This invention relates to improvements in an optical isolator which consists of a combination of a magneto-optical element and a permanent magnet, and more specifically to a small optical fiber terminal fitted with an optical isolator for such applications as semiconductor laser modules and optical amplifiers.
Optical isolators are used in optical amplifiers, semiconductor laser modules, and the like. They comprise a polarizer, a magneto-optical element that functions as a Faraday rotator, and another polarizer, all combined between a laser light source and an optical fiber terminal or between two optical fiber terminals, further combined with a permanent magnet to saturate or magnetize the magneto-optical element.
Generally, scatters in the position of light incident upon the optical isolator and in the diameter of the beam cause variations in the characteristics of the isolator. There is a demand, therefore, that the isolator characteristics remain unaffected by some changes in the incident position and the beam diameter. The scatters can be avoided if the optical isolator is combined integrally with an optical fiber terminal to provide a singular unit, since the combination establishes a constant relation between the two components, making the adjustments therebetween no longer necessary. We previously proposed an optical fiber terminal fitted with an optical isolator having this advantage in U.S. Pat. No. 5,359,689.
The optical fiber terminal fitted with an optical isolator disclosed in the above-cited U.S. Patent has, besides the afore-described advantage, the merit of being handled as simply as a mere optical fiber terminal not fitted with an optical isolator.
The invention of the prior application is characterized in that an optical fiber, a ferrule that holds the fiber in position, and an unsaturated optical isolator located on the end face of the fiber are assembled integrally so that the maximum outside diameter of the optical isolator can be either substantially equal to, or not more than twice, the outside diameter of the ferrule. However, the optical characteristics required of the sandwich consisting of a magneto-optical element and polarizers, placed in the hollow of the cylindrical magnet, restrict the decrease of its diameter, and naturally the outside diameter of the magnet cannot be decreased appreciably. For this reason the optical fiber terminal fitted with an optical isolator according to the prior application has the drawback of being unable to achieve satisfactory size reduction.
There is another related invention which does not pertain to an optical fiber terminal fitted with an optical isolator but to an optical isolator itself. Patent Application Public Disclosure No. Sho 63-123014 disposes a Faraday rotator axially outside of a cylindrical permanent magnet which is magnetized in its axial direction and saturates the Faraday rotator magnetically with the magnetic field created by the permanent magnet. The cylindrical permanent magnet may have a small inside diameter and hence the magnet too may have a small outside diameter. However, this presents a problem of increased overall length of the optical isolator. An elongated isolator, when used for a semiconductor laser module, for example, would place a limitation to the lens system. An additional disadvantage is that, because the magnetic field portion outside of the magnet is much weakened compared with the inside, a large magnet must be used to achieve magnetic saturation.
As explained above, it has hitherto been impossible to decrease both the outside diameter and length of an integrated combination of an optical isolator and an optical fiber terminal.
It is therefore an object of the present invention to provide an optical fiber terminal fitted with an optical isolator which is shorter and smaller in outside diameter than usual.
Another object of the invention is to provide an optical fiber terminal fitted with an optical isolator in which the permanent magnet can generate an increased magnetic field strength.